1. Field
One embodiment of the invention relates to a membrane switch having a flexible insulating sheet to be folded when it is used, and more specifically, to a structure for enhancing the waterproof performance of a fold-back portion of the insulating sheet. Further, the present invention relates to a keyboard in which depression of keytops is electrically detected by using the membrane switch and an electronic apparatus, such as a portable computer, having the keyboard.
2. Description of the Related Art
A keyboard used in, for example, a portable computer comprises keytops and a membrane switch. The membrane switch, which serves electrically to detect depression of each keytop, has fixed contacts and movable contacts. The fixed and movable contacts face one another across a space in positions corresponding to the keytops, individually.
A membrane switch is described in Jpn. Pat. Appln. KOKAI Publication No. 9-259693, for example. It has a flexible insulating sheet on which fixed contacts and movable contacts are printed. In this membrane switch, the fixed and movable contacts are printed on one surface of the insulating sheet. The insulating sheet is folded so that the fixed and movable contacts face one another.
According to the membrane switch of this fold type, the insulating sheet is provided with a first sheet portion on which the fixed contacts are printed, a second sheet portion on which the movable contacts are printed, and a fold-back portion situated between the first and second sheet portions.
The first and second sheet portions are located symmetrically with respect to the fold-back portion. A plurality of conductor patterns are printed on the fold-back portion. The conductor patterns cross the fold-back portion so as to span the boundary between the first and second sheet portions. The fold-back portion is folded in a U shape such that the first and second sheet portions face each other. Thus, a gap is defined inside the fold-back portion, and the conductor patterns are situated in the gap.
According to the conventional folded membrane switch, the fold-back portion of the insulating sheet is bent with a relatively large amount of curvature in order to reduce the thickness of the switch. Thus, a stress easily acts on the fold-back portion of the insulating sheet, so that fine cracks may possibly develop in the fold-back portion after prolonged use.
Depending on the working environment of the keyboard, therefore, water inevitably runs along the cracks and gets into the space inside the fold-back portion. The water confined in the gap will cause migration in the conductor patterns that are situated in the gap.
To cope with this, in the membrane switch disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-259693 mentioned before, a waterproof protective film is bonded to the fold-back portion of the insulating sheet with an adhesive agent. The adhesive agent is disposed along the outer peripheral edge portion of the protective film, and most of it is situated off the conductor patterns. The protective film covers the conductor patterns from the opposite side of the insulating sheet, thereby enhancing the waterproof performance of the conductor patterns.
According to the membrane switch disclosed in the patent document described above, however, the protective film must be bonded very carefully and elaborately so that the adhesive agent entirely fills the space between the film and the insulating sheet. Thus, operation for bonding the protective film requires a lot of time, as well as much labor and skill.
When the protective film is bonded to the insulating sheet, moreover, it conceals the adhesive agent, so that it is difficult to determine whether or not the adhesion of the film is satisfactory. In consequence, the state of adhesion of the protective film sometimes may vary according to products, possibly lowering the reliability of the membrane switch in waterproof performance.